The present invention relates to nuclear medicine. It finds particular application in conjunction with single photon emission computed tomography (SPECT) and will be described with particular reference thereto.
In early nuclear or Anger cameras, a patient was injected with a radioactive dye and a nuclear camera head was positioned stationarily over the region of interest. The nuclear camera head included a scintillation crystal which produced a flash or scintillation of light each time it was struck by radiation emanating from the radioactive dye in the subject. An array of photomultiplier tubes and associated circuitry produced an output signal which was indicative of the (x,y) position of each scintillation on the crystal.
To assure that the radiation causing each scintillation came from a known path through the patient, a collimator was placed on the patient face of the scintillation crystal. The collimator typically included a rectangular grid of lead vanes which assured that each scintillation was produced by radiation travelling along a path from the patient substantially perpendicular to the scintillation crystal face.
Other collimators were developed to magnify the region of interest. In a cone beam collimator, the vanes were tapered such that all the vanes pointed at a common focal point. Radiation reaching the scintillation crystal was constrained by the cone beam collimator to radiation travelling along divergent paths in both the x and y direction such that the entire scintillation crystal was used to examine a relatively small region of interest. This magnification improved the resolution in both planar dimensions. Rather than magnifying in two dimensions, fan beam collimators were developed which magnified in one dimension. That is, the vanes were oriented such that the vanes focused the radiation on a focal line, rather than a focal point.
The data collected by the nuclear camera was analogous to one projection view or, more accurately, one view of each of a plurality of slices of CT data. By rotating or indexing the detector head to a multiplicity of orientations circumferentially around the subject, a full set of CT data could be collected and reconstructed using conventional CT type algorithms.
Due to the great weight of the detector head and the lead collimator, a counterweight was often used to counterbalance the rotating detector head. Rather than using a passive counterweight, SPECT cameras have also been built using a second detector head positioned opposite to the first. Two oppositely disposed detector heads substantially double the data collection rate. Similarly, positioning three heads at 120.degree. intervals around the subject, the data collection rate was substantially tripled. For many examinations, the three heads are positioned substantially touching such that the patient is substantially surrounded by radiation receptive surfaces.
CT reconstruction algorithms normally call for 180.degree. of data to reconstruct a tomographic image. Thus, a single detector head needed to rotate about 180.degree. around the subject to produce a complete 180.degree. data set. A two-head camera still had to rotate 180.degree. around the subject to get a full data set, but the full data set was a 360.degree. data set. A three head camera needs to rotate about 120.degree. around the subject to generate a full 360.degree. data set.
By positioning two detector heads at 90.degree. relative to each other, a full 180.degree. of data can be collected with 90.degree. of rotation. Even although the two 90.degree. detector heads generate only a 180.degree. data set rather than a 360.degree. data set at a savings of only 30.degree. of rotation relative to a three head camera, the more rapid scanning speed is perceived as advantageous for some applications. The two 90.degree. detector heads have mechanical and data collection disadvantages.
The present invention provides a new and improved SPECT camera system which provides the advantages of two 90.degree. detector heads, as well as the advantages of a three head SPECT system.